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T-Tubule Biogenesis and Triad Formation in Skeletal Muscle and Implication in Human Diseases Lama Al-Qusairi1,2,3,4,5,6 and Jocelyn Laporte1,2,3,4,5*

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T-Tubule Biogenesis and Triad Formation in Skeletal Muscle and Implication in Human Diseases Lama Al-Qusairi1,2,3,4,5,6 and Jocelyn Laporte1,2,3,4,5* Al-Qusairi and Laporte Skeletal Muscle 2011, 1:26 http://www.skeletalmusclejournal.com/content/1/1/26 Skeletal Muscle REVIEW Open Access T-tubule biogenesis and triad formation in skeletal muscle and implication in human diseases Lama Al-Qusairi1,2,3,4,5,6 and Jocelyn Laporte1,2,3,4,5* Abstract In skeletal muscle, the excitation-contraction (EC) coupling machinery mediates the translation of the action potential transmitted by the nerve into intracellular calcium release and muscle contraction. EC coupling requires a highly specialized membranous structure, the triad, composed of a central T-tubule surrounded by two terminal cisternae from the sarcoplasmic reticulum. While several proteins located on these structures have been identified, mechanisms governing T-tubule biogenesis and triad formation remain largely unknown. Here, we provide a description of triad structure and plasticity and review the role of proteins that have been linked to T-tubule biogenesis and triad formation and/or maintenance specifically in skeletal muscle: caveolin 3, amphiphysin 2, dysferlin, mitsugumins, junctophilins, myotubularin, ryanodine receptor, and dihydhropyridine Receptor. The importance of these proteins in triad biogenesis and subsequently in muscle contraction is sustained by studies on animal models and by the direct implication of most of these proteins in human myopathies. Introduction tubules). In skeletal muscle, T-tubules tightly associate To trigger skeletal muscle contraction, the action poten- with the sarcoplasmic reticulum (SR), in a region called tial generated by motor neurons is transmitted through terminal cisternae/junctional SR. The close association motor nerves to muscle cells. The excitation-contraction of one T-tubule with two terminal cisternae on both (EC) coupling, i.e. signal transmission from the sarco- sides of the tubule forms the triad (Figure 1). lemma to the actin/myosin apparatus, is mediated by a A large set of specialized proteins takes part in EC second messenger, calcium ions. Indeed, muscle fibers coupling and includes: i) the Dihydhropyridine Receptor contain large internal calcium stores with the ability to (DHPR), a voltage gated calcium channel located on T- quickly release and retrieve calcium (Figure 1, right tubule membranes [1,2], ii) the Ryanodine Receptor panel). For a fast and fine-tuning of muscle contraction, (RyR1), a calcium release channel that is localized on these stores are maintained under the control of the the junctional face of SR and appears as “feet” when action potential, which ensures calcium release simulta- observed by electron microscopy (Figure 1, left panel) neously within the whole interior of the muscle fiber. As [3], iii) calcium buffering proteins such as calsequestrin myofibers are 50-100 μm in diameter and several milli- in the lumen of the SR [4], iv) calcium channel regula- meters to centimeters long, a highly specialized struc- tors such as calmodulin, FKBP12 and many others [5,6], ture named the triad is necessary to overcome spatial v) Sarco-Endoplasmic Reticulum Calcium ATPase limits in using calcium as secondary messenger, and pumps (SERCA), which is indirectly involved in EC cou- connect the sarcolemma with the calcium stores. The pling via its action in the rapid removal of the cytosolic sarcolemma forms regular invaginations which insert calcium after fiber shortening to replenish the calcium between myofibrils, termed transverse tubules (T- stores [7]. Noteworthy, the physical coupling between RyR1 and DHPR occurs specifically in skeletal but not * Correspondence: [email protected] in cardiac muscles and allows the transmission of the 1Department of Translational Medecine and Neurogenetics, IGBMC (Institut signal within 2 ms in skeletal muscles compared to 100 de Génétique et de Biologie Moléculaire et Cellulaire), 1 rue Laurent Fries, ms in cardiac muscles [8]. In cardiac myofiber, RyR2- 67404 Illkirch, France Full list of author information is available at the end of the article mediated calcium release is induced by extracellular © 2011 Al-Qusairi and Laporte; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Al-Qusairi and Laporte Skeletal Muscle 2011, 1:26 Page 2 of 11 http://www.skeletalmusclejournal.com/content/1/1/26 Figure 1 Triad organization in skeletal muscle. Left: Electron micrograph of a triad junction. A central T-tubule is flanked on both sides by a terminal cisternae element from the sarcoplasmic reticulum. Arrows indicate electron-dense junctional feet corresponding to the ryanodine receptor-dihydhropyridine receptor complex. Right: Schematic representation of a mammalian muscle sarcomere and surrounding membranes. T-tubules shown in gray are specialized invaginations of the sarcolemma. The elaborated sarcoplasmic reticulum network is shown in blue. Note the close proximity of T-tubules and terminal cisternae of the sarcoplasmic reticulum (adapted from [104]; © 2007 by Pearson Education, Inc.). calcium entry via DHPR in a mechanism called calcium- which results in 5-15 fold increase in the relative induced calcium release (CICR) [8]. Moreover, in fibers volume of T-tubule system is reversed spontaneously of small diameter, such as the body muscles of [11,12]. Moreover, this observation is specific to trans- Amphioxus, peripheral couplings between SR and the verse tubule membranes, as no other intracellular mem- plasmalemma have similar function to triads. In addi- brane systems appear to be involved [12], probably due tion, all differentiating muscle fibers pass through a to the fact that their lumen connects to the extracellular stage where T-tubules are not present and EC coupling space. is mediated by such peripheral couplings. In addition to this artificial condition, this vacuolation In this review, we will focus on the molecular phenomenon is observed upon muscle fatigue or dis- mechanisms underlying T-tubules biogenesis and triad eases [10,13]. Based on this plasticity and on the large formation specifically in skeletal muscle. Triad defects membrane surface of the T-tubules network which cor- linked to human monogenic diseases will also be responds to about 80% of the sarcolemma surface, sev- highlighted. eral functions non-related to EC coupling are proposed for the T-tubules system [10,13]. These include: i) water T-tubule plasticity balance and regulation of cell volume, ii) recovery from The T-tubule membrane possesses a high plasticity muscle fatigue, iii) transport pathways including endocy- which provides the stability required during muscle con- tosis, exocytosis and the penetration of foreign DNA. traction, and facilitates repair upon damage. In addition The molecular mechanisms involved in these processes to its principal function in EC coupling, the plasticity of are still to be investigated. T-tubules confers to this system non-related EC functions. Morphological aspects of triad biogenesis It has been reported that the treatment of isolated Sarcoplasmic reticulum muscle fibers with glycerol efflux-influx or with other The SR represents the main calcium store in striated low molecular weight nonelectrolytes (such as sugars) muscle. It is highly specialized to ensure the simulta- physically affects T-tubules morphology. Such osmotic neous release of intracellular calcium in the entire cyto- shock can convert the T-tubule network into many sol of the muscle cell. The first step of SR biogenesis membrane-bound vacuoles, which can either remain starts by the formation of tubular endoplasmic reticu- interconnected by normal T-tubules, or become sepa- lum (ER) (30-60 nm in diameter) adjacent to the myofi- rated (Figure 2) [9,10]. Surprisingly, this vacuolation bril [14]. Subsequently, these tubular branches of ER Al-Qusairi and Laporte Skeletal Muscle 2011, 1:26 Page 3 of 11 http://www.skeletalmusclejournal.com/content/1/1/26 Figure 2 Dynamics of T-tubule vacuolation produced by the efflux of glycerol. A single frog skeletal muscle fiber stained with a lipophilic fluorescence probe is shown. Shown are serial confocal microscopic images of the same fiber (a) 5 minutes, (b) 12 minutes and (c) 30 minutes after the fiber was transferred from a solution containing 110 mM glycerol to a solution without glycerol (reprinted from [10]; © 2001 with permission from Elsevier). develop into reticular structures surrounding the myofi- particular, between E16 and E18, when all junctions brils [15]. Finally, the newly formed SR engages cou- become filled by feet. The width of the junctional gap is plings at the A-I interface with the T-tubule originating between 9 and 12 nm. The maturation of the RyR con- from the sarcolemma. The molecular determinants taining elements is accomplished at birth [17]. Elegant implicated in the functional and structural organization experiments using tagged SR proteins in differentiating of the SR have been reviewed elsewhere [16]. myotubes showed that the SR organization was paral- The chronology of SR biogenesis was well investigated leled by a dynamic localization of longitudinal and junc- using electron microscopy (EM) during muscle differen- tional SR proteins [19]. tiation in mouse [17]. These observations were also sup- Transverse tubule ported by studies employing chicken embryo [18]. In T-tubules are invaginations of the plasma membrane, mouse,
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